A fossil of an ancient shark has gills that are more similar to a tuna or goldfish than the modern shark, an American Museum of Natural History research team has found.

Paleontologists have theorized that sharks didn't evolve much over time, but the fossil believed to be 325 million years old may change what students read in their textbooks, National Geographic reported.

While they have long been called "living fossils," sharks may actually have evolved quite a bit since ancient times. After the new findings, researchers may be examining bony fishes instead of modern sharks to outline the evolution of the shark's jaw and gills.

"We're throwing down the gauntlet," said study co-author John G. Maisey, a vertebrate paleontologist at the AMNH, as quoted by National Geographic. "This is the real condition in an early shark, and it changes how we have to think about the evolution of jawed vertebrates."

When it comes to their gills, "modern sharks are in this respect evolutionarily quite advanced," Maisey said.

The team's research, which has been published in the journal Nature, marks the first time an ancient shark's gills have been so closely studied. Found in the Ozark Mountains of Arkansas, the rare fossil holds the gill arches of Ozarcus mapesae, a newly discovered shark species with gill structure similar to both ancient and modern fish.

"The basic arrangement we found in Ozarcuswould characterize the common ancestor of all these forms," said Maisey, as quoted by National Geographic.

Scientists may struggle with further study since shark and fish fossils from the time period dated to 325 million years ago are extremely rare, and the concept of the shark as a "living fossil" isn't going anywhere yet.

 "It's an entrenched idea," said Matt Friedman, a paleontologist at the University of Oxford in the U.K., according to National Geographic. "You see it in textbooks."

But at least advances in technology like the high-resolution computer modeling used in this study will help scientists analyze fossils in the future. 

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